A fiber optic probe for measuring the reflectance spectrum has been used for analyzing the state of pigments in the surface layer of a living body by measuring the light irradiating and reflected from the surface of the organism. Such fiber optic probe comprises a fiber bundle including light illuminating optical fibers and light receiving optical fibers in which a plurality of optical fibers formed of transparent materials such as plastic or glass are bundled in parallel. The fiber bundle is enclosed by a cover on the outer periphery of the bundle. Such a fiber bundle used as a measuring probe is known from, for example, Japanese Utility Model Laying Open No. 59-113749.
The optical fiber generally comprises a core and a jacket covering the core, with the diameter of the core being several 10 .mu.m and the diameter of the jacket or cladding being about 150 .mu.m.
The above mentioned Japanese Utility Model Laying Open No. 59-113749 discloses a metal sleeve holder provided at the outermost periphery so as to protect the probe, and to facilitate the use of the probe. However, what is disclosed in that reference is a number of common bundled optical fibers fiber each consisting of a core and a cladding. The diameter of cladding is about 100 .mu.m and the diameter of the core is about several 10 .mu.m, in a generally used plastic fiber or a glass fiber.
FIG. 1 is an enlarged view of a portion of a conventional probe. FIG. 2 shows a reflectance spectrum with distortions measured by the conventional probe of FIG. 1. FIG. 3 is a graph showing the relationship between the blood volume and the optical density in a sample area.
The conventional probe shown in FIG. 1 includes an illuminating light supplying optical fiber 1 and a light receiving optical fiber 2 arranged next to each other. The light supporting illuminating optical fiber 1 comprises a core 1a and a jacket or cladding 1b , while the light receiving optical fiber 2 comprises as a core 2a and a jacket or cladding 2b. Each of the cores 1a and 2a has a diameter of about several 10 .mu.m and each of the jackets 1b and 2b has a diameter of about 100 .mu.m. Therefore, the distance l between the centers of the illuminating light supply optical fiber 1 and the light receiving optical fiber 2 is about 100 .mu.m.
Now, when a portion of a tissue such as a liver containing much blood, is to be measured by the conventional probe, the reflectance spectrum measured should be as shown by the dotted line in FIG. 2. However, actually the measured result is as shown by the solid line. The reason for this is as follows. As shown by (1) in FIG. 3, the optical density linearly increases as the blood volume increases. When the wavelength of the illuminating light becomes shorter, the effect of scattering increases, so that the increase of the optical density becomes non-linear and is distorted as shown at (2) in FIG. 3. Such a measuring organ probe is not suitable for measuring functions, for example, of the liver containing much blood.